1. Field of the Invention
The present invention relates to a method for blocking the dislocation propagation of a semiconductor.
2. Description of the Related Art
Currently, common solid state semiconductor devices comprise light emitting diodes (LED), laser diodes, and semiconductor RF (radio frequency) devices. Generally, the light emitting diode, the laser diode, or the semiconductor RF device includes a semiconductor device such as an RF device and a light emitting device with a p-n junction formed on the epitaxial substrate.
The structure of a blue or red light emitting diode comprises a sapphire substrate, a buffer layer formed on the sapphire substrate, an N-type GaN semiconductor layer, an active layer partly covering the N-type GaN semiconductor layer, a P-type GaN semiconductor layer formed on the active layer, and two contact electrodes respectively formed on the aforesaid two semiconductor layers.
The luminous efficiency of LEDs is affected by some factors such as internal quantum efficiency and external quantum efficiency. The major factor affecting the internal quantum efficiency is the amount of dislocation existing in the active layer. However, lattice mismatch always occurs in the materials of a sapphire substrate and a GaN layer. Therefore, the threading dislocation also occurs during the epitaxial process.
FIG. 1A is a schematic diagram of a prior art disclosed by Taiwanese patent publication No. TW561632. The device comprises a sapphire substrate 10, an N-type semiconductor layer 11 formed on the sapphire substrate 10, an active layer 12 capable of generating light with default wavelengths formed on the N-type semiconductor layer 11, and a P-type semiconductor layer 13 formed on the active layer 12.
A plurality of recesses 14 are periodically arranged and formed on the sapphire substrate 10 using photolithography equipment and a reactive ion etching process. Therefore, the N-type semiconductor layer 11 on the sapphire substrate 10 without crystal defects is filled in each of the recesses 14. The depth and width of each of the recesses 14 are respectively 1 μm and 10 μm. The pitch, defined as the distance between the centers of two adjacent recesses, is 10 μm.
A Bell, R. Liu, F. A. Ponce, H. Amano, I. Akasaki, D. Cherns, et al. propose a GaAlN doped with Mg grown on a patterned sapphire substrate in Applied Physics Letters, vol. 82, No. 3, pp. 349-351 and discuss its luminous characteristics and microstructure. This paper puts forth that a sapphire substrate patterned with a plurality of grooves is formed by performing photolithography and reactive ion etching processes, and an epitaxial layer of GaAlN doped with Mg is formed on the patterned sapphire substrate. A great deal of threading dislocation exists in the epitaxial layer, and the epitaxial lateral overgrowth (ELOG) regions of the epitaxial layer above each of the stripe-shaped grooves have no defects.
In addition, Shulij Nakamura, Masayuki Senoh, Shinichi Nagahama, Naruhito Iwasa, Takao Tamada, et al., researchers of Nichia Chemical Co., propose a laser diode of modulation-doped strained-layer superlattices formed on an epitaxial lateral overgrowth GaN substrate in Appl. Phys. Lett. 72(2), 211-213. Referring to FIG. 1B, a buffer layer 91 and a GaN layer with a thickness of 2 μm are sequentially formed on a sapphire substrate 90. A silica layer 93 with a thickness of 0.1 μm is further formed on the GaN layer 92, and the silica layer 93 has stripe-shaped windows 94 with a width of 4 μm by a photolithography process. The windows 94 of the silica layer 93 act as a mask (silica mask) on the GaN layer 92. Finally, an N-type GaN layer 95 is sequentially formed on the mask by a two jet-flow MOCVD method so that a sequential device can be formed thereon.
The researchers of Nichia utilize an amorphous mask to direct epitaxial vertical overgrowth layers in the windows 94 to be laterally merged with each other so as to form epitaxial lateral overgrowth layers on the mask. Accordingly, the dislocation density of the epitaxial layer is reduced.
Although the aforesaid prior patents and techniques can reduce the threading dislocation occurring in an epitaxial layer during an epitaxial process, all of the aforesaid recesses and windows are regularly arranged, so the dislocation defects cannot be completely eliminated.
Finding a means to reduce the dislocation density of an epitaxial layer during an epitaxial process and improving the luminous characteristics of the epitaxial layer are still critical issues for the manufacturers currently investing significant resources on research in the epitaxial field.